Composition of the volcanic sedimentary breccia at the Carrapateena iron-oxide-copper-gold deposit, SA

Abstract

The Carrapateena Iron Oxide Copper Gold (IOCG) deposit is located on the eastern margin of the Gawler Craton. Economic Cu mineralisation is hosted by the Carrapateena Breccia Complex (CBC) which sits within in the Donington Granite. The Volcanic Sedimentary Breccia Unit (VSB) of unknown origin lies within the CBC and is considered to be uneconomically mineralised. Preliminary investigation suggests that the VSB was deposited after the main mineralisation event at Carrapateena and may therefore provide important geological constraints on the age, evolution and preservation of the ore system. This study focuses on six samples from the VSB. Multiple techniques were used to investigate the mineralogy and composition of the VSB, including; MAIA mapping, optical microscope petrography, SEM, SEM-CL and SEM-MLA. The VSB can be described as a matrix-supported sedimentary breccia/conglomerate with clasts of angular quartz, rounded volcanics, granite and lithics. The VSB is dominated by fine chlorite with quartz and chlorite making up over 64% of the VSB by area. In order to constrain the age of the VSB, in-situ U-Pb geochronology and trace element analysis was undertaken on constituent zircon, monazite and rutile. The concordant U-Pb zircon analyses resulted in two populations of ages with modern day Pb loss trends. Zircons with higher Th/U correspond to a younger population with a weighted 207Pb/206Pb age of 1619 ± 10 Ma, correlating to the age of the St Peter Suite. Zircons with lower U/Th ratios correspond to the age of the Donington Suite granite host rock at ca. 1855 Ma. Monazite from the non-mineralised samples were also shown to reflect the age of the host Donington suite granite. Monazites from the mineralised sample are interpreted to be hydrothermal in origin and have a range of ages from 1200 to 2100 Ma with a concentration around 1660 Ma, indicating the potential for multiple mineralisation or hydrothermal remobilisation event(s).